Hi I'am trying to simulate compressible cavitating flow, there are various papers I have read stating you need to have the beta and beta* coefficients as variables of the mach number. Now I have tried using CEL functions to do this but get an error saying that the coeffs must be calculated to a constant value (and thus cannot vary). Is there any way to get around this?

But there is always a way. Have a look in the VARIABLES file, I think it is in the /etc/ directory. Find the beta and beta* variables and remove the bit forcing them to be constant values. There is a chance this will allow it, but be aware the limitation was put in there for good reason so there is a high probability this will not work.